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Experts said diesel drivers are subsidising businesses who qualify for loyalty fuel cards and benefit from big discounts.

Lorries and trucks used by firms to transport goods tend to use diesel, as it is more efficient and better for carrying heavy loads.

Some retailers have been better at passing on savings to motorists than others.

Asda offers the cheapest petrol and diesel.

It has cut diesel prices by 1.94 p over the last month to 107p. The price drivers pay includes the wholesale price, fuel duty VAT and the profit made by the retailer.

Petrol drivers have fared better, with retailers passing on the 2p fall in wholesale costs over the last month after fierce pressure from campaigners and consumer groups

But BP petrol stations - which are either owned by the oil giant or independent – have cut prices by just 1.5 p to 113.09p.

This makes it £3.35 more expensive to fill up at tank at a BP station than at an Asda forecourt.

According to the AA, this means there are huge differences in the price motorists from different areas of the country pay at the pumps – depending on which retailers operate in their town or village.

Fuel retailers were slammed by campaigners for failing to pass on a fall in wholesale costs in the aftermath of the Brexit vote on June 23.

They claimed retailers were taking advantage of Brexit, and the expectations of motorists that the shock vote would cause oil prices – and therefore fuel prices - to rise.

The criticism was compounded by the fact that millions of people are thought to have taken their summer holidays in the UK because of the fall in the value of the pound.

According to the AA, there are huge differences in the price motorists from different areas of the country pay at the pumps

Although a price war finally kicked off between the supermarkets towards the beginning of the month, campaigners said diesel drivers appear to have been left behind.

Howard Cox, founder of the campaign group FairFuelUK said:

'The murky cartel driven world of fuel pricing continues to fleece motorists of the fairest deals at the pumps. Notably diesel drivers are victims of their continuing greed with retail margins rising early August. This is why the chancellor must set up a monitoring body to achieve fair and transparent pricing at the pumps. Millions of drivers and small businesses are shouting for pricing clarity, which it seems the Westminster out of touch politicos continue to ignore.'

A BP spokesman said: 'We aim to price competitively in all of our petrol stations'.